RFID (Radio Frequency Identification) is a powerful technology that, due to its numerous advantages, is supposed to replace the various identification systems such as barcodes or magnetic stripes in a short time. There are three devices involved in an RFID protocol: a Reader, a Tag, and a back-end Database. One of the key factors for the RFID technology is that, in order to be used on large scale, the price of the Tags has to be cheap: it cannot be expensive because who is supposed to use it would need a great amount of them, furthermore, Tags must have very small dimensions. The low-cost nature of such devices implies the impossibility to use standard cryptographic protocols on them, furthermore, Ultra-Lightweight Tags even lack the necessary computational power to generate random numbers. Many experts are trying to build secure protocols that involve just simple bitwise logical operations for these Tags, but, unfortunately, each of these protocols turned out to be vulnerable to some serious attack after short time from publication. The need for a secure RFID authentication protocol today seems more urgent than ever, because this technology is already used for security purposes, such as electronic passports. The challenge to build a secure protocol for Ultra-Lightweight RFID systems is so hard that in several years no one has been able to build one. In this thesis we analyze in great detail a recently proposed protocol for Ultra-Lightweight RFID systems called SLAP, with the aim of finding new vulnerabilities. SLAP has already been violated (along with a set of similar protocols) by Safkhani and Bagheri, that have recently published a de-synchronization attack. At the end of our analysis, we will propose an impersonification attack to the protocol, along with a fix for our attack and some considerations on the attacks proposed on this kind of protocols.